This is for anyone that doesn't like when the RPM's that are slow to drop or even rise slightly when you push the clutch in to shift. Another symptom, is that when cruising in a higher gear and you let your foot off the gas, the car almost cruises on its own for a bit.

First, understand the problem. The engine is recieving too much air through the IAC valve. From my knowledge Ford did this for emissions reasons.

As a side effect of this fix, when you downshift, the car burbles a lot more, sounds like a carburated engine. You get a little more engine braking effect.

The FIX:
Most older mustangs have an external IAC valve. Meaning that it is easy to find, follow a small air hose (1/2" ID) to a silver cylinder thing on your intake (the IAC solenoid). Once you found it, the older fix was to buy a 1/2" copper pipe cap, drill out a small hole, insert this into the line the restrict air flow through the IAC, and your done.
The Mach has a slightly different setup.
We have an INTERNAL IAC air supply. Meaning that there is no hose leading to it.

Next, look between your intake and the firewall, there should be a silver cylinder shaped object with 2 bolts leading to your intake.

Remove the two bolts and move this assembly.

You should see the following, the IAC assembly has two "ports" with the solenoid controlling the air flow between these two ports, what we want to accomplish is to restrict the airflow leading through this.

To do this, there should be a small gasket on your intake, or it could be stuck to the IAC valve.

Remove this (carefully)

Trace out the shape of this gasket as well as the holes for mounting bolts, and the two airholes, onto a piece of 1/32" aluminum or something similar.

Cut out the outline of this "restrictor plate" from the aluminum.

Cut out the holes for your bolts to go through.

Cut out a single one of the Air holes.
Now the tricky part. You have to decide how big to make the other hole because this determines the amount of air you are restricting.

The smallest you should go would be around 5/32", the worst case if you go to small is your car will stall when fix is complete, if this happens just take the plate out, make the hole bigger, and check that.
The hole I used is around 9/32" which is about the medium you would want to go.

After your "restricter plate" is cut out, mount it with the small hole on the passenger side of the IAC, and make sure to replace the gasket against the intake.
Start your car, you should notice the RPMs drop when you start back to normal very quickly.
If it idles smoothly, try putting turning on the AC, this will load the engine down.
If your car stalls out, remove the plate, make the hole bigger, try again.
If still idling smoothly, bolt your shaker back on, take it for a spin. You might need to adjust your driving/clutching style after being used to the crappy RPM hang, so don't be discouraged.
Also, you might want to disconnect your battery during the procedure to make your computer relearn it tweaks after you finish.

If, after you start the car you notice a "whistling" sound, then take a look at your plate, make sure the BIG hole is at least as big as the hole on the IAC valve. ALso make sure edges of holes are rounded (sharp edges are bad), and the holes are round. This happened on mine, and I fixed it.

I also got a plate from magnum 347 ...works well and downshifting actually slows me down now. Decelleration is a lot louder as mentioned - more pops and bangs. Does take a little driving time to get used to it. All in all, definetly worth it if your hanging rpms are annoying you.

i would just get a lighter flywheel. why restrict the air to your motor?

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because that wouldn't work. Even if u loose weight there, IAC is sending more air to slow down the drop.
With this plate, ur fooling ECU with no side effect.
I know this, because I tried changing Flywheel method on my 99 Cougar. It did speed up the drop, but not much at all. little did I know that all i needed was simple copper plug instead of $500+install that didn't work.
On the other hand, after doing copper plug mods, with flywheel swapped out, it came down too fast, that sometimes it stolled

Founding Member

I used the copper pipe method and I removed it. I have a light flywheel and the car would idle ok with the A/C and all, but when I would pop it out of gear on the road, it would stall out. The RPM's would drop so fast that the engine couldn't regain when it dropped below 1000.

I also got a plate from magnum 347 ...works well and downshifting actually slows me down now. Decelleration is a lot louder as mentioned - more pops and bangs. Does take a little driving time to get used to it. All in all, definetly worth it if your hanging rpms are annoying you.

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Works as I said... Another satisfied customer... Now they are brushed aluminum plates that are ready to ship....

I bought one of these from magnum too. It works, and the small price is worth it to not have to make one yourself, even though it's not that difficult. Some people seem to have a problem with him making them and selling them, but it's only a few bucks folks, and if he wants to take the time and effort to make them for people, I say more power to him and he can have my 5 bucks.

Much Agreed! Magnum sent me one of the plates also and what a difference! I LOVE the sound between shifts now..sounds more like a muscle car than it did before...I love to shift gears with a honda beside me and watch the look on there face. But seriously this make a great deal of improvement and it makes shifts alot better becuase you get a nice stiff engage and not a slip feel like with the old set up. It is worth the 5 bux guys and by the time he buys parts he is only going to make a buck or two so order up

Founding Member

Since more unburned fuel could make its way to the catalytic convertors, think it would pose a threat to their longevity? I'm guesssing no, since the 5.0 motors (and many other motors while I'm at it) didn't have a hanging rpm issue.

Also, anyone getting a better jump off the line with this mod? Some of the Mach 1 guys were claiming so.

Since more unburned fuel could make its way to the catalytic convertors, think it would pose a threat to their longevity? I'm guesssing no, since the 5.0 motors (and many other motors while I'm at it) didn't have a hanging rpm issue.

Also, anyone getting a better jump off the line with this mod? Some of the Mach 1 guys were claiming so.

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My throttle honestly feels a little more snappy in terms of throttle response...I love the way the car feels now..its alot more responsive and fun to drive